1. Field of the Invention
This invention relates to machinery for folding sheet material and, more particularly, to method and apparatus for folding sheet material in forming envelopes and the like.
2. Description of the Prior Art
In automated envelope forming operations, it is well known to utilize at one of the envelope forming stations a folding mechanism which folds a web of sheet material or blanks cut in preselected lengths. A common folding mechanism includes plow-share-type folding guides which extend a preselected length along the envelope feed line. The web of sheet material or cut blanks are conveyed through the plowshare folder by a vacuum table or a combination of overlying conveyors and pinch rolls.
U.S. Pat. No. 5,094,658 discloses a plowshare folder in an envelope forming machine where individual envelope blanks are conveyed through the folding mechanism by a vacuum table. The vacuum table includes a series of spaced apart conveyor belts that are driven over the surface of a vacuum table in the direction of feed of the blanks. The vacuum table includes a plate having apertures which draw in air as a result of a vacuum created by the evacuation of air from a vacuum plenum created below the surface of the vacuum plate. The plowshare folder includes a thin wall of rigid material having at the front end an initial planar surface which gradually bends in a 180.degree. turn. Through the 180.degree. turn, blades of the plowshare engage the seal flap region of the envelope blank. The flaps to be folded extend parallel to the axis of movement of the blank across the vacuum table and are folded into overlying relation with the main body of the blank.
As further disclosed in U.S. Pat. No. 5,094,658 a pair of plowshares are oppositely positioned in the feed line to simultaneously fold opposite side flaps. This operation is routinely performed in the formation of large open end envelopes.
U.S. Pat. No. 4,994,010 also discloses a machine for forming large open end envelopes having clasps to maintain the seal flaps of the envelope closed. Prior to attaching the clasp the lateral flaps of the blank are folded into overlapping relation to form the back side of the envelope. Then the clasp is attached to the folded flaps.
The folding operation in U.S. Pat. No. 4,994,010 is performed by a plowshare-type folding mechanism that includes fold loops and folding blades. During the folding operation, the lateral flaps converge and are glued together in an overlapping area by means of an adhesive layer which has been previously applied with the result that the folded and overlapping flaps form the back side of the envelope. The lateral flaps are simultaneously folded by folding blades which have an inner curvature that evolves from an initial point where the side flaps extend horizontally to a point where the flaps are folded in a tubular configuration. The guide surfaces of the folding blades curve progressively through 180.degree. along travel of the blank.
The progressive folding of side flaps of an envelope blank in a plowshare-type folder is accomplished by the envelope blank advancing through zones of the folding blades. The curvature of the blades progressively changes from 0.degree. to 180.degree. through zones that extend in the direction of the feed line. The blades have a width which corresponds to the full width of the flap being folded. Consequently the entire surface of the flap being folded is in contact with the blades through the 180.degree. folding.
During the progressive folding, the flap moves from a horizontal position at the 020 position through an angle of 180.degree. to a position horizontally in overlying relation on the body of the blank. At the downstream end of the folding operation, the flap is moving downwardly to the 180.degree. position while at the upstream end the flap is moving upwardly to the 90.degree. position.
Over its entire length the flap is required to move in different directions during the folding operation. The sheet material forming the flap has a natural tendency to resist a change in direction of the fold. This resistance to folding introduces stress into the blank, making it difficult to maintain the fold aligned with the score line where the fold is to take place.
The nature of sheet material to resist bending or folding increases as the length of the blank being folded increases. U.S. Pat. Nos. 1,851,061 and 2,054,832 disclose plowshare-type folding mechanisms for envelope making machines in which the sheet material is folded before it is cut into segregated blanks for forming envelopes. The plowshare blades disclosed in these patents contact the full width of the flap being folded. At one point in the folding operation, the flap is bent upward toward 90.degree. and at another point the flap is folded downwardly toward 180.degree.. Particularly for an extended length of sheet material being folded, the sheet material resists folding which can result in misalignment of the fold at the score line.
In U.S. Pat. No. 2,077,952, individual blanks are cut from a continuous web and then folded by the provision of upwardly inclined converging arms which serve to turn the side flaps through approximately a quadrant. Downstream of the converging arms a pair of belts act on the flaps. Active stretches of the belts are twisted from substantially upright to horizontal positions to fold the flaps into overlying relationship with the body of the envelope blank.
With the envelope making machine disclosed in U.S. Pat. No. 2,077,952 and the other patents discussed above, the folding mechanisms attempt to support the entire width of the flap being folded. Consequently folding forces are directed at the free edge of the flap a distance substantially removed from the score line where the fold is to be formed. Particularly for envelopes with large flaps to be folded, the folding process commences at even a greater distance from the score line. Therefore, the greater the width of material in contact with the folding device and the further the folding operation commences from the score line, the greater the difficulty in maintaining the fold aligned with the score line.
Therefore, there is need in sheet folding machinery, such as envelope forming machines, for apparatus that initiates the folding operation and applies the folding forces closely adjacent to the score line. The free edge of the flap should be supported as the fold is accomplished without applying the folding forces to the flap free edge.